This invention relates to a two-terminal photo sensor including an integrated circuit having a photo diode and a photo current amplifier, and especially relates to such a photo sensor wherein the amplifier exhibits a highly stable gain; i.e. linear sensitivity.
A two terminal integrated circuit is disclosed in the W. Gontowski patent application Ser. No. 296,748, filed Aug. 27, 1981, and assigned to the same assignee as is the present invention. That two terminal photo sensor may be employed in the same basic circuit and in the same way that a simple discrete photo diode 1 would be as illustrated in FIG. 1, e.g. the DC supply 2 is connected in series with the photo diode 1 and a resistor load 3. The current drawn at any time by the two terminal sensor is proportional to the ambient light intensity, and so will be the amplitude of voltage dropped across the load resistor.
It would clearly be advantageous to employ a high voltage supply and a large value resistor to provide a relatively high signal level across the load for a given light level. However, a large signal voltage means a large swing of voltage across the two terminals of the photo sensor. Under these conditions, nonlinearities in photo sensor sensitivity occur that are attributable to Early effect in the output transistors in each current mirror stage. When many such stages are employed, there is a cumulative Early effect that leads to a substantial difference in the photo sensor sensitivity at low light levels relative to the sensitivity at high light levels. In other words the output current (and sensitivity) becomes nonlinear with ambient light level.
The Early effect is normally manifested by the current mirror circuits. That is to say, the output current varies a bit with changes in output voltage. This is because of the slight variation of V.sub.BE with collector voltage (typically -0.001 volts per volt).
An exception, however, is the Wilson current mirror circuit, shown in FIG. 2, and further described in U.S. Pat. No. 3,588,672 issued June 28, 1971, that provides good output current stability with load voltage variations. This is achieved by means of a cascode transistor 10 that holds substantially constant the collector voltage of the input transistor 12. Thus the "current-gain" controlling base-emitter junctions of the input transistor 12 and output transistor 15 have a current-density-determined voltage drop that is not subject to a change in the supply voltage, because the collectors of both transistors 12 and 15 are clamped to a low and relatively invarient value. But unfortunately the Wilson current mirror circuit is not suitable for amplification, namely by having disparate "gain controlling" base-emitter junction areas.
It is also known to add a cascode circuit 17 to a simple current mirror circuit 19 as shown in FIG. 3 in order to prevent large swings in amplifier output voltage from affecting the output current 20. The classical cascode circuit of FIG. 4 adds to a grounded emitter amplifier transistor 22 a "grounded base" cascode transistor 24. Here again the Early effect is in fact defeated, although the cascode clamping of the amplifier transistor collector was originally intended to obviate Miller effect, i.e. collector-base capacitance coupling that degrades high frequency performance.
It is an object of the present invention to provide a two-terminal integrated-circuit photo sensor with more stable sensitivity over a wide range of light levels and over a corresponding wide range of signal current levels in the integrated-circuit photo sensor terminals.
It is yet a further object of this invention to provide such a photo sensor that retains its linear response when operating with a low DC supply voltage across the two terminals.